Characterizations of critical processes in liquid-liquid phase separation of the elastomeric protein-water system: microscopic observations and light scattering measurements

Molecular Serum Albumin Unmask Nanobio Properties of Molecular Graphenes in Shungite Carbon Nanoparticles

Serum albumin is a popular macromolecule for studying the effect of proteins on the colloidal stability of nanoparticle (NP) dispersions, as well as the protein-nanoparticle interaction and protein corona formation. In this work, we analyze the specific conformation-dependent phase, redox, and fatty acid delivery properties of bovine albumin in the presence of shungite carbon (ShC) molecular graphenes stabilized in aqueous dispersions in the form of NPs in order to reveal the features of NP bioactivity. The formation of NP complexes with proteins (protein corona around NP) affects the transport properties of albumin for the delivery of fatty acids. Being acceptors of electrons and ligands, ShC NPs are capable of exhibiting both their own biological activity and significantly affecting conformational and phase transformations in protein systems.

Coarse-grained model of tropoelastin self-assembly into nascent fibrils

Elastin is the dominant building block of elastic fibers that impart structural integrity and elasticity to a range of important tissues, including the lungs, blood vessels, and skin. The elastic fiber assembly process begins with a coacervation stage where tropoelastin monomers reversibly self-assemble into coacervate aggregates that consist of multiple molecules. In this paper, an atomistically based coarse-grained model of tropoelastin assembly is developed. Using the previously determined atomistic structure of tropoelastin, the precursor molecule to elastic fibers, as the basis for coarse-graining, the atomistic model is mapped to a MARTINI-based coarse-grained framework to account for chemical details of protein-protein interactions, coupled to an elastic network model to stabilize the structure. We find that self-assembly of monomers generates up to ∼70 ​nm of dense aggregates that are distinct at different temperatures, displaying high temperature sensitivity. Resulting assembled structures exhibit a combination of fibrillar and globular substructures within the bulk aggregates. The results suggest that the coalescence of tropoelastin assemblies into higher order structures may be reinforced in the initial stages of coacervation by directed assembly, supporting the experimentally observed presence of heterogeneous cross-linking. Self-assembly of tropoelastin is driven by interactions of specific hydrophobic domains and the reordering of water molecules in the system. Domain pair orientation analysis throughout the self-assembly process at different temperatures suggests coacervation is a driving force to orient domains for heterogeneous downstream cross-linking. The model provides a framework to characterize macromolecular self-assembly for elastin, and the formulation could easily be adapted to similar assembly systems.

Pressure-Induced Dissolution and Reentrant Formation of Condensed, Liquid-Liquid Phase-Separated Elastomeric α-Elastin

We investigated the combined effects of temperature and pressure on liquid-liquid phase separation (LLPS) phenomena of α-elastin up to the multi-kbar regime. FT-IR spectroscopy, CD, UV/Vis absorption, phase-contrast light and fluorescence microscopy techniques were employed to reveal structural changes and mesoscopic phase states of the system. A novel pressure-induced reentrant LLPS was observed in the intermediate temperature range. A molecular-level picture, in particular on the role of hydrophobic interactions, hydration, and void volume in controlling LLPS phenomena is presented. The potential role of the LLPS phenomena in the development of early cellular compartmentalization is discussed, which might have started in the deep sea, where pressures up to the kbar level are encountered.

Coacervation of tropoelastin

The coacervation of tropoelastin represents the first major stage of elastic fiber assembly. The process has been modeled in vitro by numerous studies, initially with mixtures of solubilized elastin, and subsequently with synthetic elastin peptides that represent hydrophobic repeat units, isolated hydrophobic domains, segments of alternating hydrophobic and cross-linking domains, or the full-length monomer. Tropoelastin coacervation in vitro is characterized by two stages: an initial phase separation, which involves a reversible inverse temperature transition of monomer to n-mer; and maturation, which is defined by the irreversible coalescence of coacervates into large species with fibrillar structures. Coacervation is an intrinsic ability of tropoelastin. It is primarily influenced by the number, sequence, and contextual arrangement of hydrophobic domains, although hydrophilic sequences can also affect the behavior of the hydrophobic domains and thus affect coacervation. External conditions including ionic strength, pH, and temperature also directly influence the propensity of tropoelastin to self-associate. Coacervation is an endothermic, entropically-driven process driven by the cooperative interactions of hydrophobic domains following destabilization of the clathrate-like water shielding these regions. The formation of such assemblies is believed to follow a helical nucleation model of polymerization. Coacervation is closely associated with conformational transitions of the monomer, such as increased β-structures in hydrophobic domains and α-helices in cross-linking domains. Tropoelastin coacervation in vivo is thought to mainly involve the central hydrophobic domains. In addition, cell-surface glycosaminoglycans and microfibrillar proteins may regulate the process. Coacervation is essential for progression to downstream elastogenic stages, and impairment of the process can result in elastin haploinsufficiency disorders such as supravalvular aortic stenosis.

Protein-based aqueous-multiphasic systems

This paper reports the formation of aqueous-multiphasic systems (AMPS) exclusively made using elastin-like polypeptides (ELP) which have the ability to undergo reversible inverse phase transitions. Manipulating variables such as the salt concentration and the molecular weight and the composition of ELPs (using different amino acid sequences or by fusing the ELP with different functional proteins) permits modulation of the temperature at which phase transition takes place, the number of phases that are formed, and the composition of the multiple aqueous phases. Using these variables, isotropic hybrid colloids with tunable functionality (in this case, fluorescent intensity) and anisotropic colloids with variable morphologies could be generated. While formation of AMPS and anisotropic colloids has been reported in the literature using synthetic polymers, to our knowledge this is the first report of generating such systems using proteins.

Fibrillins, fibulins, and matrix-associated glycoprotein modulate the kinetics and morphology of in vitro self-assembly of a recombinant elastin-like polypeptide

Elastin is the polymeric protein responsible for the properties of extensibility and elastic recoil of the extracellular matrix in a variety of tissues. Although proper assembly of the elastic matrix is crucial for its durability, the process by which this assembly takes place is not well-understood. Recent data suggest the complex interaction of tropoelastin, the monomeric form of elastin, with a number of other elastic matrix-associated proteins, including fibrillins, fibulins, and matrix-associated glycoprotein (MAGP), is important to achieve the proper architecture of the elastic matrix. At the same time, it is becoming clear that self-assembly properties intrinsic to tropoelastin itself, reflected in a temperature-induced phase separation known as coacervation, are also important in this assembly process. In this study, using a well-characterized elastin-like polypeptide that mimics the self-assembly properties of full-length tropoelastin, the process of self-assembly is deconstructed into "coacervation" and "maturation" stages that can be distinguished kinetically by different parameters. Members of the fibrillin, fibulin, and MAGP families of proteins are shown to profoundly affect both the kinetics of self-assembly and the morphology of the maturing coacervate, restricting the growth of coacervate droplets and, in some cases, causing clustering of droplets into fibrillar structures.

Thermoresponsive self-assembly of short elastin-like polypentapeptides and their poly(ethylene glycol) derivatives

Short polypeptides with four pentad repeats, (VPGVG)(4) and (VPAVG)(4), were synthesised by manual fluorenylmethoxycarbonyl/tert-butyl (Fmoc/t-Bu) solid phase peptide synthesis using a convergent approach. In the next step, the peptides were coupled via their N-terminus with activated semi-telechelic poly(ethylene glycol) O-(N-Fmoc-2-aminoethyl)-O'-(2-carboxyethyl)undeca(ethylene glycol) (Fmoc-PEG-COOH) to yield monodisperse Fmoc-PEG-peptide diblock copolymer. Both the presence of the terminal hydrophobic Fmoc group and the hydrophilic PEG chain in the copolymers were shown to play a crucial role in their self-associative behaviour, leading to reversible formation of supramolecular thermoresponsive assemblies. The peptides and their PEG derivatives were characterised by HPLC, NMR and MALDI-TOF MS. The associative behaviour of the peptides and their PEG derivatives was studied by dynamic light scattering, MAS NMR and phase contrast microscopy. [image: see text]

Aqueous two-phase system formation kinetics for elastin-like polypeptides of varying chain length

The kinetics of aqueous two-phase system (ATPS) formation for elastin-like polypeptides (ELP) with defined chemical composition and chain length was investigated by dark field microscopy in an on-chip format with a linear temperature gradient. Scattering intensities from peptide solutions in the presence and absence of sodium dodecyl sulfate (SDS) were recorded as a function of temperature and time, simultaneously. It was found that the formation of the ATPS for three ELPs of different molecular weights (36 075, 59 422, and 129 856 Da) in the absence of SDS followed a coalescence mechanism, and the rate constant and activation energy were independent of chain length. With the introduction of SDS into the ELP solutions, the rate constants were attenuated more strongly with increasing chain length. Moreover, the coalescence process in the presence of SDS showed non-Arrhenius kinetics as a function of temperature. For the two shorter ELPs, ATPS formation occurred via coalescence at all SDS concentrations and temperatures investigated. On the other hand, the coalescence process was greatly suppressed for the longest ELP at elevated temperatures and higher SDS concentrations. Under these circumstances, ATPS formation was forced to proceed via a mixed Ostwald ripening and coalescence mechanism.

pH-Induced structural transitions during complexation and coacervation of beta-lactoglobulin and acacia gum

pH-Induced structural changes during complex coacervation between beta-lactoglobulin (BLG) and Acacia gum (AG) in aqueous solutions were determined by coupling slow in situ acidification of BLG/AG mixed dispersions and different experimental methods. The combined signal evolution of dynamic light scattering at 90 degrees scattering angle (I(90)), electrophoretic mobility, turbidimetry (tau), circular dichroism, and phase contrast microscopy allowed the distinction of critical structural transitions and the definition of their corresponding pH. The formation of soluble BLG/AG complexes was initiated at pH(sc) (4.90), since I(90) and tau significantly increased from the baseline. In parallel or just following complexation, a conformational change of BLG was detected at pH(pct) (4.8). An increase in positive charge density of BLG induced complex aggregation at pH(ca) (4.7). More efficient charge neutralization of aggregated complexes, especially through the lowering of the number of AG negative charges, promoted initiation of phase separation at pH(psi) (4.4). Mixed dispersions became unstable and phase separation occurred at pH(ps) (4.2). The phase separation of mixed dispersions was suggested by the maximum value of scattered light, by an important acceleration of the dispersion turbidity, by a strong increase of hydrodynamic radii, and by the first appearance of light fluctuations as observed by phase contrast microscopy. At the microscopic level, the first coacervates were observed at pH(coa) (4.0), near the pH of the maximum of turbidity. It was also noticed that, from the onset of interactions between biopolymers, the pH decrease led to (i) a gradual homogenization of particle size in the mixed dispersion as suggested by the decrease of dispersion polydispersity and (ii) conformational transitions of the protein (a loss of alpha-helix structure at pH(pct) and a gain in protein secondary structure near pH(coa), probably involving beta-sheet components).

Probing the mechanism of aqueous two-phase system formation for alpha-elastin on-chip

The kinetics of formation of the aqueous two-phase system (ATPS) for alpha-elastin was studied by dark field microscopy in an on-chip linear temperature gradient. Scattering intensities of protein solutions were recorded as a function of temperature and time, simultaneously at several concentrations. It was found that the formation rate of the ATPS could be fit as a first-order process and that the apparent rate constant increased with protein concentration. The activation energy for the process was 9.5 +/- 0.5 kcal/mol, and this result was consistent with a coalescence mechanism. Experiments were also conducted with varying concentrations of sodium dodecyl sulfate, which shut off the coalescence mechanism forcing ATPS formation to proceed through Ostwald ripening. When this was done, the activation energy increased to 33 +/- 2 kcal/mol and the kinetics became consistent with a second-order process.

Mechanism for the Phase Transition of a Genetically Engineered Elastin Model Peptide (VPGIG)40in Aqueous Solution

The concentration dependence of the pressure- and temperature-induced cloud point transition (Pc and Tc, respectively) of aqueous solutions of an elastin-like polypeptide with a repeating pentapeptide Val-Pro-Gly-Ile-Gly sequence (MGLDGSMG(VPGIG)40VPLE) was investigated by using apparent light scattering, differential scanning calorimetry, and circular dichroism methods. In addition, the effects of salts and surfactants on these properties were investigated. The Pc and Tc of the present peptide in aqueous solution were strongly concentration dependent. The calorimetric measurements showed that the enthalpy of transitions was 300-400 kJ/mol, i.e., 7-10 kJ/mol per VPGIG pentamer. The Tc of the (VPGIG)40 solution was highly affected by the addition of inert salts or SDS. The effects of salts were consistent with those observed in the lyotropic series or Hoffmeister series. The CD spectrum at low peptide concentrations indicated that the present peptide forms type II beta-turn-like structure(s) at higher temperatures, but the temperature dependence of random coil diminishment (195 nm) and beta-turn formation (210 nm) were not exactly coincident. A hypothetical mechanism of the (VPGIG)40 phase transition that could account for these observations was postulated. Observations suggest that the temperature-responsive properties of the elastin model peptides occur via a mechanism involving conformational change-association-aggregation and that the first two are strongly interactive.

Thermodynamic and Hydrodynamic Properties of Human Tropoelastin

Tropoelastin is the soluble precursor of elastin that bestows tissue elasticity in vertebrates. Tropoelastin is soluble at 20 degrees C in phosphate-buffered saline, pH 7.4, but at 37 degrees C equilibrium is established between soluble protein and insoluble coacervate. Sedimentation equilibrium studies performed before (20 degrees C) and after (37 degrees C) coacervation showed that the soluble component was strictly monomeric. Sedimentation velocity experiments revealed that at both temperatures soluble tropoelastin exists as two independently sedimenting monomeric species present in approximately equal concentrations. Species 1 had a frictional ratio at both temperatures of approximately 2.2, suggesting a very highly expanded or asymmetric protein. Species 2 displayed a frictional ratio at 20 degrees C of 1.4 that increased to 1.7 at 37 degrees C, indicating a compact and symmetrical conformation that expanded or became asymmetric at the higher temperature. The slow interconversion of the two monomeric species contrasts with the rapid and reversible process of coacervation suggesting both efficiently incorporate into the coacervate. A hydrated protein of equivalent molecular weight modeled as a sphere and a flexible chain was predicted to have a frictional ratio of 1.2 and 1.6, respectively. Tropoelastin appeared as a single species when studied by pulsed field-gradient spin-echo NMR, but computer modeling showed that the method was insensitive to the presence of two species of equal concentration having similar diffusion coefficients. Scintillation proximity assays using radiolabeled tropoelastin and sedimentation analysis showed that the coacervation at 37 degrees C was a highly cooperative monomer-n-mer self-association. A critical concentration of 3.4 g/liter was obtained when the coacervate was modeled as a helical polymer formed from the monomers via oligomeric intermediates.